The present invention relates to a method for making stationary heat transfer fields visible by photochemical means by applying a photographic film layer to the surface of an article and exposing the film layer by conducting a gas over it.
Methods are known for making stationary heat transfer zones visible which are based on the analogy between mass and heat transfer. An example of one such method is the so-called equidensity method using Agfa Contour Film.RTM. (Kimmel, H. and Opitz, H., Siemens Review, Vol. XL, 6/73, pages 275 to 278). For this purpose, a uniformly moistened photo-gelatin layer is applied to the surface of an article to be investigated which is exposed to the flow of dry air thereover. As a result of evaporation to the flowing air, the film is transformed into locally different degrees of dryness corresponding to the stationary local state of the boundary layer flow.
From the residual quantities of liquid differently distributed locally in the photo-gelatin film after the exposure of the film to the flow, a photographic half-tone picture (photogram) can be developed, the density distribution of which represents the residual moisture distribution in the photo-gelatin layer due to the existence of a moisture-density curve. By reason of the above-mentioned analogy between the mass and heat transfer, the evaporated amounts of liquid represent a measure for the latter, and a definite local heat transfer coefficient can be correlated with a given local photographic density in the photogram. An absolute determination of the numerical values has not, however, been possible to date, i.e., one must still calibrate with physically determined values. The half-tone photogram obtained by the above-mentioned method represents the stationary mass transfer distribution of a planar subassembly as it is produced by the stationary thickness distribution of the Prandtl boundary layer, controlled by evaporation.
"Equidensity lines" can be constructed from half-tone photograms. "Equidensities" are areas or lines of equal, arbitrarily selectable photographic density tones which are displayed black on white or vice-versa. If these "equidensities" are given certain colors, the correlation of the equidensities among each other is facilitated substantially in complicated situations. The equidensities obtainable with wet-film techniques are lines (areas) of equal residual moisture, so-called "isohumids".
The equidensities are prepared by means of so-called "contour film".RTM. (Agfa). By varying the exposure time, the "window" in the contour film can be shifted over the entire density range of the half-tone original. Depending on the density steps (controllable by simultaneously copying a step grey wedge), several transparent equidensity separations are obtained in this manner which can be stacked with proper registration (sandwich assembly) and copied together. By chromogenic development of the black-and-white separations, color equidensities can be obtained as slides. However, the preparation of the equidensities by this method is time-consuming and requires the use of Agfa Contour Film.RTM..